Many beneficial devices or structures in a myriad of applications rely on batteries as a power source. As shown in FIG. 1, illustrative liquid-cell battery 101, is characterized by an electrolyte liquid 102 which provides a mechanism for an electrical charge to flow in direction 103 between a positive electrode 104 and a negative electrode 105. When battery 101 is inserted into an electrical circuit 106 with illustrative load 108, it completes a loop which allows electrons to flow uniformly in direction 107 around the circuit 106. The positive electrode thus receives electrons from the electrical circuit 106. These electrons then react with the materials of the positive electrode 104 in reduction reactions that generate the flow of a charge to the negative electrode 105 via ions in the electrolyte liquid 102. At the negative electrode 105, oxidation reactions between the materials of the negative electrode 104 and the charge flowing through the electrolyte fluid 102 result in surplus electrons that are released to the electrical circuit 106.
As the above process continues, the active materials of the positive and negative electrodes 104 and 105, respectively, eventually become depleted and the reactions slow down until the battery is no longer capable of supplying electrons. At this point the battery is discharged. It is well-known that, even when a liquid-cell battery is not inserted into an electrical circuit, there is often a low level reaction with the electrodes 104 and 105 that can eventually deplete the material of the electrodes. Thus, a battery can become depleted over a period of time even when it is not in active use in an electrical circuit. This period of time will vary depending on the electrolyte fluid used and the materials of the electrodes.
Batteries having at least one nanostructured surface have been recently proposed wherein nanostructures are used to separate the electrolyte from the electrode, by employing certain so-called electrowetting principles, until such a time that the battery is to be used. An example of the use of electrowetting principles applied to batteries is described in copending U.S. patent application Ser. No. 10/716,084 filed on Nov. 18, 2003 (hereinafter the “'084 Application”) and entitled “Electrowetting Battery Having Nanostructured Surface,” which is hereby incorporated by reference herein. As disclosed in the '084 Application, when it is desired that the battery generate a charge, the electrolyte is caused to penetrate the nanostructured surface and to come into contact with the electrode of the battery, thus resulting in the above-discussed flow of electrons around a circuit. Such a penetration of nanostructures is achieved, for example, by applying a voltage between the nanostructures and the electrolyte such that the contact angle of the electrolyte relative to the nanostructured surface is decreased. When the -contact angle is decreased, the electrolyte penetrates the nanostructures and is brought into contact with the electrode.
Thus, nanostructured batteries (also known as “microbatteries”) offer distinct advantages over prior art liquid-cell batteries, for example, in terms of performance and may hold further advantages in terms of battery type and application. For example, lithium (Li) ion batteries are well-known battery components which supply power to a number of electronic devices (e.g., mobile telephones, cameras and laptop computers, to name just a few). As is well understood, lithium ion batteries generate current due to the flow of lithium ions between battery electrodes.
As is also well-known, carbon serves as an excellent electrode material for batteries. In particular, depending upon the crystalline structure of the carbon such material serves at least two different functions: (1) so-called “glassy carbon” which is characterized by sp2 type chemical bonding (where each carbon atom is bonded to three other carbons as in crystalline graphite) but has no long-range graphitic crystal structure. For example, well-known phenol-formaldehyde (novalac) photoresist patterns form glassy carbon patterns, which can serve as a current collector electrode due to electrically conductive and relatively chemically inert properties; and (2) so-called “turbostratic carbon” which is characterized by a random orientation of graphitic domains, the term “turbostratic” indicating a type of crystalline structure where the basal planes have slipped sideways relative to each other, causing the spacing between planes to be greater than ideal. As is well-known, graphitic domains intercalate lithium ions thereby serving as an excellent electrode material for lithium ion batteries. Such graphitic domains of carbon may be formed, as is well understood, from thermal decomposition of certain polymer materials such as poly(acrylonitrile) and poly(p-phenelyene).
Carbon exhibits different properties in a solid state as a function of the type of bonding between individual carbon atoms. For example, diamond and graphite are well-known carbon structures exhibiting different material properties. In diamond, each carbon atom is bonded to four other carbon atoms, such that each carbon is centered in a tetrahedron and surrounded by four carbon atoms located at the vertices. To establish these four equivalent bonds, the electrons in the “s” and “p” orbitals hybridize thereby forming sp3 bonds. In contrast, graphite consists of planar sheets of carbon atoms configured in a hexagonal array where each carbon atom is bonded to three other carbons through sp2 hybridization of orbitals. This type of bonding creates mechanically soft and electrically conductive materials (as contrasted with diamond, as discussed above, which forms a mechanically hard and electrically insulating material). In addition, the space between carbon sheets is relatively large, thereby enabling intercalation of other elements (e.g., lithium).
In the case where planar sheets of carbon atoms extend over long distance (i.e., greater than 1000's of atoms), the materials are considered crystalline. In glassy carbon materials, only the nearest neighbors may be co-planar. Between crystalline graphite and glassy carbon, lie a range of morphologies where the size of the planar, sp3 bonded, domains varies. For example, turbostratic graphite is a known material where the sheet size is relatively small (greater than 5 nanometers). In terms of lithium battery applications, depending upon the degree of order in the sheets they may be layered in a manner that is random with respect to both translation as well as rotation (see, for example, T. Zheng et al., “Effect of turbostratic disorder in graphitic carbon hosts on the intercalation of lithium”, in The American Physical Society, Physical Review B, Volume 51, Number 2, January 1995, which is hereby incorporated by reference herein (hereinafter “Zheng”)). In Zheng, supra, it is shown that the ordering of turbostratic carbon layers is essential for achieving significant lithium intercalation. Further, Zheng has correlated lithium ion battery performance (i.e., capacity) with ordered layer stacking within turbostratic carbon domains.
Additional work, for example, by C. Wang et al., “C-MEMS Technology for Li Ion Microbatteries”, in The Electromechanical Society, Inc., Abs. 445, 206th Meeting, 2004, which is hereby incorporated by reference herein (hereinafter “Wang”), explores the use of glassy carbons in the formation of lithium ion batteries. In Wang, supra, a technique is described which forms carbon patterns from pyrolysis of certain commercially available photoresist materials such as SU-8. As is well-known, SU-8 is based on a resin available from Shell Chemical, Epikote 157, CAS#28906-96-9, which is a bisphenol-A based multi-functional array. As such, as will be well understood, the various material properties of such a resin allow for the formation of a glassy polymer that, in turn, form glassy, amorphous carbon upon pyrolysis. Wang employs such properties to fabricate and characterize completed microbatteries with posts of C-MEMS doped with lithium for anodes and dodecylbenzenesulfonate-doped poly(pyrrole) (PPYDBS) for cathodes. Wang claims advantages such as an increase in the total available surface area for intercalation by using high aspect ratio posts, and creation of posts directly using a photosensitive precursor to carbon. However, despite such advantages, the efficiency of microbatteries based on intercalating lithium into glassy carbon is relatively low.
Therefore, the availability of graphitic nanostructured batteries which can be used to deliver more efficient (for example, in terms of power, size and life) lithium microbatteries would be advantageous.